1. Field of Invention
The present invention relates to a noise reduction system, and more particularly relates to a noise reduction system used in the BTSC system.
2. Description of Related Art
In the 1980's, the United States FCC (Federal Communications Commission) adopted new regulations covering the audio portion of television signals that permitted television programs to be broadcast and received with bi-channel audio. In those regulations, the FCC recognized and gave special protection to a method of broadcasting additional audio channels that is also called the BTSC (Broadcast Television System Committee) system. The BTSC system defines MTS (multi-channel television sound) transmission and its audio processing requirements.
FIG. 1 shows a noise reduction system of the prior art. The noise reduction system 110 is used in the BTSC system to reduce noise of an audio signal and generate an encoded audio signal during an encoding process in the approach of the digital processing. The noise reduction system 110 has an audio spectral compressing unit 120, a wideband compression circuit 150, and a multiplier 160 when the noise reduction system 110 is used in the encoding process. The audio spectral compressing unit 120 has a filter 130 and a memory 140. The filter 130 filters an input signal according to a transfer function, a variable d, and several parameters (coefficients of the transfer function). The transfer function is:
                              H          ⁡                      (            z            )                          =                                                            b                0                                            a                0                                      +                                                            b                  1                                                  a                  0                                            ⁢                              z                                  -                  1                                                                          1            +                                                            a                  1                                                  a                  0                                            ⁢                              z                                  -                  1                                                                                        (        1        )            
The memory 140 is arranged to store the parameters. When the variable d is greater than zero, i.e. d>0, the memory 140 outputs the parameters (b0/a0)+, (b1/b0)+ and (a1/a0)+ to the filter 130; when the variable d is less than zero, i.e. d<0, the memory 140 outputs the parameters (b0/a0)−, (b1/b0)− and (a1/a0)− to the filter 130.
From this transfer function (1), the memory 140 needs to store 6 parameters (b0/a0)+, (b1/b0)+, (a1/a0)+, (b0/a0)−, (b1/b0)− and (a1/a0)−. Because the cost of the memory is proportional to the capacity of the memory, a noise reduction system with a memory of smaller capacity is needed.